RANCHO BERNARDO  Balloons, streamers and signs decorated the lobby of the Personally Fit gym on Monday to celebrate the 100th birthday of its oldest member Margaret McKenzie, but the peppy centenarian hardly took time for the chocolate cake and birthday song. She had a workout to finish.

For the past 18 years, the San Diego native has put in three 90-minute workouts a week at the Rancho Bernardo gym, and when she’s not exercising there, she’s climbing the stairs and walking the paths around the Remington Club, the nearby retirement community where she lives. McKenzie doesn’t have a secret for her longevity, but she thinks healthy eating and exercise are key.

“I don’t know,” she said, during a workout break Monday. “Not everyone as careful as I am lives to be 100. I am very healthy, but I guess I’m just lucky.”

Gym co-owner Brian Gillespie said McKenzie is an inspiration to others because she virtually never misses a workout, and she’s so dedicated to improving her fitness.

“Sometimes when you sit down at that age, you don’t get back up,” Gillespie said. “She’s an all-around great person and she’s still going strong.”

McKenzie was born Oct. 6, 1913, and grew up in San Diego’s Kensington neighborhood. She was a sickly child. Although she survived two bouts with flu and pneumonia during the 1918-’19 Spanish flu epidemic, her body was so ravaged by illness that all of her hair fell out. In adolescence, she was constantly absent from school because of chronic colds.

Her health improved as a young adult when she started walking to work (in high heels) each day. In her late 20s, she became a follower of nutrition author Adelle Davis, who preached the benefits of eating lean proteins, whole grains, fresh vegetables, vitamins and supplements.

In 1937, she met her husband, Reginald, on a blind date in Pasadena, where she worked for the Board of Equalization and he worked as an accountant for tire-maker B.F. Goodrich. The couple traveled a great deal for his job in their first years of marriage, but when daughters Marilyn and Madelyn came along in the mid-1940s, he went to work for Aerojet, a startup missile-maker that would eventually make the rockets that took man to the moon. A highlight of their marriage, McKenzie recalls, were invitations to two state dinners at the White House with presidents Johnson and Nixon.

When Reginald died 30 years ago of a heart attack at age 70, McKenzie was committed to living on her own for as long as she could. She kept golfing (a sport she took up in her 50s) until she was 92. She drove a car until three years ago. And she still plays bridge three or four days a week.

She rides a shuttle to the gym every Monday, Wednesday and Friday morning, where she works on balance and strength exercises with her personal trainer, Sean Thompson.

“She’s my idol,” said Thompson, 32, who began working with McKenzie in January of last year. He said she can do some exercises nearly twice as fast as clients who are 30 years her junior.

“She is the perfect client. I can count the times she has missed a session on one hand,” he said. “She’s my oldest client, of about 30, and she’s the only one I have to tell all the time to slow down.”

Although she’s had both hips replaced and she suffered a torn shoulder muscle in a fall earlier this year, McKenzie said she feels guilty if she’s not out walking for at least a half-hour each day. She also looks forward to her sessions with Thompson, who pushes her to improve each day.

“He’s very nice and patient with me,” she said. “And he always tells me if it isn’t hard, we’re not getting any place.”